Add Python Raw memory helper functions
Add Python Raw memory helper functions

In Python 3.4+, Raw memory helper functions were added to wrap, track,
and check `malloc` and `free` calls for C memory outside of the GIL.
These have the same API as the existing `PyMem_*` calls except they use
Raw and never require the GIL. For Python 2/3 compatibility, export the
existing `PyMem_` functions on Python 2 as `PyMem_Raw` where they act
basically equivalently. These are especially important as Python 3.6
changed the existing `PyMem_` functions to allocate from pymalloc
instead of the C allocator meaning the GIL must now be held with them.
So these are Python 2/3 alternatives that can be relied on to not
require the GIL.

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